vasovagal syndromevasovagal triggersdefine vasovagalneurocardiogenic syncope syndromeneurocardiogenic syncope diagnosisvasovagal syncopal attackrecurrent vasovagal syncopemalignant vasovagal syndromevasovagal syncope pathophysiologyvasovagal syncope pregnancyvasovagal syncope diagnosisvasovagal syncope exercise